Research On The Propagation Process Of Solar Energetic Particles

When a solar flare or a coronal mass ejection erupts on the Sun,a sudden increase in the flux of solar energetic particles(SEPs)with energies ranging from tens of ke V to tens of Me V or even Ge V can be observed usually.This increase in the flux of SEPs caused by solar activity is called the SEP event.SEP events have adverse effects on space activities.The study on SEP events can promote the understanding of the acceleration and propagation mechanism of SEPs in the heliosphere.SEP transport models can provide the prediction of SEP events thus reducing the threat on space activities.The solar energetic proton event occurred on 2019 April 21,which was observed by the Parker Solar Probe(PSP)and ACE simultaneously,is simulated in this thesis.We obtain a good simulation result in terms of fitting the numerical result to the observation,thus giving the diffusion coefficients of SEPs in this event.In addition,we also study the SEP event,which was observed by the PSP at a distance of 0.17 AU from the Sun,occurred on 2019 April 4.We simulate the propagation process of SEPs by considering the observation of PSP as the source of SEPs.The source of SEPs is prescribed by an assumed function in most of simulation studies on SEP events because there are no measurements of the distribution function of SEPs near the Sun.As the PSP gets closer to the Sun,one can use the in-situ observations of SEP flux as sources to study the propagation process of SEPs.The Parker field of solar wind is a heliospheric magnetic field model obtained by assuming the velocity of solar wind is uniform.This model is widely used for studying the propagation of SEPs under quiet solar wind conditions.Researchers have paid more and more attention to investigate the propagation process of SEPs in a more realistic solar wind magnetic field in recent years,although the simplified Parker field has achieved great success in studying SEP events.In this thesis,the magnetic foot-point random walking magnetic field model proposed by Giacalone is used to simulate the solar wind magnetic field at 1 AU.It is found that the amplitude and frequency of the random fluctuation of the magnetic field at 1 AU are smaller than the observation by comparing the simulated magnetic field to the observed one provided by the Wind spacecraft.Therefore,Giacalone's model needs further revision.Recently,some research based on PSP observation found that the solar wind has a strong random tangential velocity(up to 50 km/s)near the sun(within 50 solar radii),which indicates that the magnetic field lines also experience a random walk near the sun.Therefore,we use the tangential velocity to redefine the function of the random fluctuation of solar wind magnetic field in Giacalone's model,according to which we adjust the parameters of the model,so that we can obtain a similar solar wind magnetic field observed by the Wind spacecraft at 1 AU.With more and more observations measured by the PSP,the source function of SEPs can be obtained based on the PSP observation results,and the interplanetary background magnetic field can be provided based on the Giacalone's model and the parameters provided in this thesis.In the future,we will add the results of this thesis to the numerical model of the propagation of SEPs,so as to provide a more realistic numerical model for subsequent simulation work.